• Applying benefit-cost analysis, we determined the savings in morbidity, mortality, and costs of mumps vaccination in the United States. Using reported mumps incidence rates in a model cohort of 1 million persons followed up for 30 years, mumps vaccination would prevent more than 74,000 cases of mumps and three deaths. Approximating the actual incidence rate of mumps, by assuming that 90% of people are infected by age 30 and 60% of these have had clinical illness, mumps vaccination would prevent more than 540,000 cases of mumps and 23 deaths. A mumps vaccination program, in which mumps was given as part of a measles-mumps-rubella combination, would reduce costs associated with mumps by more than 86%, with a benefit-cost ratio of 7.4:1, using reported incidence rates. The program has a benefit-cost ratio of 39:1 when approximations of actual mumps incidence are used in the analysis. Mumps vaccination is highly cost beneficial.